The invention relates to a method of liquefaction of natural gas comprising the separation of hydrocarbons heavier than methane.
The natural gas and the other gaseous streams rich in methane are available generally at sites remote from the places of utilization and it is therefore usual to liquefy the natural gas in order to convey it by land carriage or by sea. The liquefaction is widely practised currently and the literature and the patents disclose many liquefaction processes and devices. The U.S. Pat. Nos. 3,945,214; 4,251,247; 4,274,849; 4,339,253 and 4,539,028 are examples of such methods.
It is also known to fractionate the streams of light hydrocarbons, for example containing methane and at least one higher hydrocarbon such as a ethane to hexane or higher through cryogenics.
Thus the U.S. Pat. No. 4,690,702 discloses a method in which the batch of hydrocarbons under high pressure (P.sub.1) is cooled so as to cause the liquefaction of one portion of the hydrocarbons; one separates a gaseous phase (G.sub.1) from a liquid phase (L.sub.1); one expands the gaseous phase (G.sub.1) to lower its pressure to a value (P.sub.2) lower than (P.sub.1) one carries the liquid phase (L.sub.1) and the gaseous phase (G.sub.1) under the pressure (P.sub.2) into a first fractionating zone, for example a purification-contact refrigeration column; one draws off at the head a residual gas (G.sub.2) rich in methane the pressure of which is then raised to a value (P.sub.3); one draws off at the bottom a liquid phase (L.sub.2) one carries the phase (L.sub.2) into a second fractionating zone, for example a fractionating column; one draws off at the bottom a liquid phase (L.sub.3) enriched with higher hydrocarbons, for example C.sub.3 +; one draws off at the head a gaseous phase (G.sub.3); one condenses at least one part of the gaseous phase (G.sub.3) and one carries at least one part of the resulting condensed liquid phase (L.sub.4) as an additional feed to the head of the first fractionating zone. In this process the second fractionating zone operates at a pressure (P.sub.4) higher than the pressure of the first fractionating zone, for example 0.5 MPa for the first zone and 0.68 MPa for the second zone.